Components that are exposed to hot corrosive gases, like for example gas turbine components which are exposed to the hot and corrosive combustion gas, are typically made of superalloys which offer a high strength at elevated temperatures. Such superalloys are usually based on nickel (Ni), cobalt (Co) or iron (Fe). Although superalloys show high strength at temperatures up to 1200° C. and more they need to be protected from oxidation and/or corrosion. This protection is typically provided by an alumina scale which is either formed in the surface of the superalloy component or by so called MCrAlY-overlay coatings, in which M stands for nickel, cobalt or iron and Y represents yttrium (Y), hafnium (Hf), or a rare earth element. During the preparation of the overlay coating the aluminium is oxidized and forms a tightly adherent scale of alumina which protects a component from oxidation and/or corrosion. Such coatings are, for example, known from EP 0 486 489 B1, EP 0 786 017 B1, or EP 1 306 456 A1. From U.S. Pat. No. 4,615,864 it is known that addition of hafnium, yttrium or lanthanum (La) to the MCrAlY composition is beneficial because the oxides of these materials assist in pinning the alumina scale.
A coating may also be called Thermal Barrier Coating (TBC) if it is intended to provide thermal insulation, e.g. of turbine aerofoils of gas turbines.